Opto-electronic systems may implement one or more feedback loops to achieve certain operational characteristics. For example, a single-frequency laser can use a passive feedback loop to correct drifts in the laser frequency to achieve a stabilized output frequency. A typical feedback loop of this type includes a photosensor to convert a fraction of the laser output into an electrical signal and a frequency reference device to generate a benchmark frequency to which the laser frequency is locked. A non-zero error signal indicative of the frequency difference between the laser frequency and the benchmark frequency is generated by the feedback loop. The error signal is used to actively maintain the laser frequency near or at the benchmark frequency. A regeneratively mode-locked laser is another example of an opto-electronic system with a passive feedback loop in which a mode beat signal from a laser is detected by a photodetector in the feedback loop. The signal from the photodetector is subsequently amplified and fed back to modulate the laser.
In addition to passive feedback loops, an active opto-electronic feedback loop may also be used in an opto-electronic system. An active feedback loop not only can provide a feedback to alter the operation of the system but also can generate and sustain an electromagnetic oscillation in the loop. Photonic oscillators can be formed by using one or more active opto-electronic feedback loops to generate radio frequency (RF) oscillations.
RF oscillators are widely used for generating, tracking, cleaning, amplifying, and distributing RF carriers and can have important applications in communication, broadcasting, and receiving systems in the radio frequency spectral range. In particular, voltage-controlled RF oscillators with phase-locked loops are used for clock recovery, carrier recovery, signal modulation and demodulation, and frequency synthesizing.
Photonic components and devices can be used to construct RF oscillators. Unlike conventional RF devices and other electronic devices which transfer information by flow of electrons, photonic technology uses photons to transfer information. In particular, photonic technology offers a number of advantages including: low loss, light weight, high carrier frequency, high security, remote capability, and immunity to electromagnetic interference.
Rapid advances in photonic fields including diode laser systems, photodetectors, electro-optical light modulators, and optical fiber systems allow implementation of photonic technology in traditional RF systems for many applications, with enhanced performance. This trend presents significant advantages for photonic RF systems and greatly expands the horizon of photonic applications. For example, optical waves may be used as a carrier to transport the information contained in RF signals through optical fibers to remote locations in a photonic RF system. This also allows some of the RF signal processing functions such as signal mixing, antenna beam steering, and signal filtering, to be accomplished optically.
One implementation of the photonic RF oscillator is an opto-electronic oscillator ("OEO") which has one or multiple active feedback loops to generate both optical modulation and electrical oscillation in radio frequency spectrum. An OEO usually has an electro-optic modulator pumped by a laser and at least one active opto-electronic feedback loop which provides in-phase feedback to an RF input port of an electro-optic light modulator. For an OEO with a single opto-electronic feedback, the open loop gain in the loop should be greater than unity in order to generate an RF electrical oscillation. The pump laser is usually a single-mode laser. The continuous photon energy from the laser is converted into RF or microwave signals by the feedback loop.
The feedback loop in an OEO can be electrical and/or optical, allowing both signal output and signal injection in either electrical or optical format or a combination thereof. An OEO can be used to produce spectrally pure RF oscillations with excellent stability, frequency tunability and low phase noise. The high performance and adaptability for both optical and electrical domain make OEOs suitable to a variety of applications for photonic communication and data processing systems. Detailed information on opto-electronic oscillators can be found, for example, in U.S. patent application Ser. No. 08/510,064, now U.S. Pat. No. 5,723,856, for a single-loop OEO by Yao and Maleki, and U.S. patent application Ser. No. 08/693,798, now U.S. Pat. No. 5,777,778, for a multiple-loop OEO by Yao.
Another type of opto-electronic oscillator suitable for generating RF oscillations is a coupled opto-electronic oscillator ("COEO"). This device directly couples a laser oscillation in an optical feedback loop to an electrical oscillation in an opto-electronic feedback loop. The laser oscillation and the electrical oscillation are intimately correlated with each other so that both the modes and stability of one oscillation are coupled with another oscillation. The optical feedback loop has a gain medium and a loop gain greater than unity to effect the laser oscillation. The coupling between two feedback loops is achieved by controlling the loop gain of the optical loop by an electrical signal generated by the opto-electronic feedback loop.
One advantage of the COEO is that a single-mode RF oscillation can be achieved without a RF bandpass filter or additional feedback loops. Another advantage is that a multi-mode laser can be used. In addition, such a coupled OEO can be used to produce ultrashort pulses. See, U.S. patent application Ser. No. 08/937,892, entitled "COUPLED OPTO-ELECTRONIC OSCILLATOR", filed on Sep. 25, 1997 by Yao.